This invention relates to a solid state noncontacting keyboard employing a differential transformer element and more particularly to a keyboard having a movable actuator for changing the mutual inductance in the primary and secondary windings of the differential transformer element.
In the prior art, keyboards that were used typically in computers, computer terminals, calculators, control panels, telephone devices, etc., were of the mechanical types and more recently solid state noncontacting types. The mechanical types suffered from mechanical wear, corrosion of contacts, etc., which limited their useful life and reliability. The solid state types, which were generally of the electromagnetic, capacitive, photoelectric or Hall Effect types, suffer from various problems including high cost, marginal performance, power consumption, susceptibility to environmental factors, and sometimes reliability. The switch concept described in this invention represents an advancement in the art of keyboard switches in terms of improved performance and reliability, and reduced cost.
The instant invention includes an X-Y matrix and in connection with this matrix, each switch embodies a differential transformer. The basic advantages of this switch concept, stem from its simplicity, its very low cost, high reliability, and due to the high degree of transformer balance that can be maintained on a printed circuit board, a high degree of switch performance in terms of a high on/off ratio or dynamic range is provided.
The prior art did not appreciate the structure, function or advantages of the instant invention. For example, in U.S. Pat. No. 3,668,697, the switch requires a network or element external to the switch to maintain its balance. Other U.S. Patents which deal generally with the subject but fail to indicate the instant invention include Nos. 3,573,369, 3,740,746, 3,745,536 and certain of my U.S. Patents such as Nos. 3,612,241, 3,638,221, 3,638,222, 3,698,531, 3,825,909, 3,836,910 and 3,918,051.
In the preferred embodiment of the present invention, the keyboard is comprised of an array of differential transformer switches, each switch consisting of four identical printed circuit coils, a switch assembly incorporating a switch stem, a return spring, a magnetic core and a key top. When a keyswitch is depressed the magnetic core will be positioned in the center of a primary-secondary coil pair, one half of the differential transformer element, changing the differential transformer from a balanced or "off" state to an unbalanced or "on" state. The two primary coils of the differential transformer element are connected in series opposing and the secondaries in series aiding. When a signal is applied to an undepressed switch the signal will not appear at the output terminals of the switch, since the differential transformer is in a balanced state. Depressing the switch unbalances the transformer permitting a drive or excitation signal to be coupled through to the output terminals of the switch. The two primary coils are printed on one side of a printed circuit board and directly opposite on the other side of the board are printed the secondary coils, with a hole provided through the board through the center of one primary-secondary pair for insertion of the magnetic actuator. The switch assembly or mechanism containing the switch keytop, magnetic core, etc., is then mounted directly above the differential transformer coils. The network which interconnects the secondary coils is printed on the printed circuit board at the same time as the coils. The primary coils and interconnecting network are printed in the same manner.
In the preferred embodiment the keyboard switch array is interconnected and organized into an X-Y matrix in which the primary coils are arranged into columns or drive lines along an X axis, and the secondary coils are arranged into rows or sense lines along a Y axis. This matrix can then be scanned by conventional means for encoding of switches and implementing various multi-key rollover circuits, etc. When implementing an N-Key rollover circuit employing the differential transformer type switch, diodes for isolation purposes are not required as is often the case with this type circuit. The differential transformer type switch is a self-contained and independent type switch, and does not require another switch, external components or circuitry to maintain its balance or perform its function.